morphology and properties of thermoplastic polyurethane nanocomposites effect of organoclay structur
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Morphology and properties of thermoplastic polyurethanenanocomposites: Effect of organoclay structure
F. Chavarria, D.R. Paul*
Department of Chemical Engineering and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712-1062, USA
Received 27 June 2006; received in revised form 23 August 2006; accepted 27 August 2006
Available online 25 September 2006
Abstract
A series of alkyl ammonium/MMT organoclays were carefully selected to explore structureeproperty relationships for thermoplastic poly-
urethane (TPU) nanocomposites prepared by melt processing. Each organoclay was melt-blended with a medium-hardness, ester-based TPU,
while a more limited number of organoclays was blended with a high-hardness, ether-based TPU. Wide-angle X-ray scattering, transmission
electron microscopy, particle analysis, and stressestrain behavior were used to examine the effects of organoclay structure and TPU chemical
structure on morphology and mechanical properties. Specifically, the following were observed: (a) one long alkyl tail on the ammonium ion
rather than two, (b) hydroxy ethyl groups on the amine rather than methyl groups, and (c) a longer alkyl tail as opposed to a shorter one leads
to higher clay dispersion and stiffness for medium-hardness TPU nanocomposites. Overall, the organoclay containing hydroxy ethyl functional
groups produces the best dispersion of organoclay particles and the highest matrix reinforcement, while the one containing two alkyl tails pro-
duces the poorest. The two TPUs exhibit similar trends with regard to the effect of organoclay structure. The high-hardness TPU nanocompo-
sites showed a slightly higher number of particles and clay dispersion. The organoclay structure trends are analogous to what has been observed
for nylon 6-based nanocomposites; this suggests that polar polymers like polyamides, and apparently polyurethanes, have a relatively good
affinity for the polar clay surface; and in the case of polyurethanes, the high affinity of the matrix for the hydroxy ethyl functional groups
in the organoclay aids clay dispersion and exfoliation. 2006 Elsevier Ltd. All rights reserved.
Keywords: Thermoplastic polyurethane; Nanocomposites; Melt processing
1. Introduction
Research on polymer layered silicate nanocomposites has
been intense over the last few years as a result of the potentially
superior properties that these materials can exhibit compared to
conventional composites. Numerous studies have shown thatthe addition of a very low percentage of layered silicates can
lead to a significant enhancement in many properties, such as
stiffness and strength [1e3], flame retardancy [4,5], gas barrier
properties [6], ionic conductivity [7,8], thermal stability [9],
and tunable biodegradability [4]. All these properties make
these materials interesting prospects for a wide variety of
applications, e.g., automotive, electronics, food packaging,
biotechnology, and others.
Polymer layered silicate nanocomposites are typically
made from organoclays composed of sodium montmorillonite
(MMT) cationically exchanged with alkyl ammonium surfac-
tants that expand the interlayer distance of the clay and makeit more organophilic. The key to obtain significant property
enhancements is to disperse, or exfoliate, the individual sili-
cate platelets within the polymer matrix to take advantage of
their high aspect ratio and surface area. The affinity of the
polymer with the surface of the clay and/or with the organic
surfactant of the organoclay is essential to promote favorable
interactions between these species and, hence, to obtain high
levels of exfoliation.
The affinity between the polymer matrix and the organo-
clay is determined, to some extent, by the polarity of the* Corresponding author. Tel.: 1 512 471 5392; fax: 1 512 471 0542.
E-mail address: [email protected] (D.R. Paul).
0032-3861/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2006.08.067
Polymer 47 (2006) 7760e7773www.elsevier.com/locate/polymer
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polymer and the type of organic modifier used to form the or-
ganoclay. Recent studies from our laboratory have shown that
a polar polymer such as nylon 6 (PA-6), exhibits a higher level
of exfoliation with an organoclay based on a surfactant with
one alkyl tail, while nanocomposites made from non-polar
polymers, such as polypropylene, polyethylene, and various
copolymers show completely opposite trends, i.e., organoclaysbased on surfactants with two or more alkyl tails lead to more
exfoliated nanocomposites than a one-tailed organoclay [10e
13]. Since a one-tailed organoclay provides less coverage of
the silicate surface than a two-tailed organoclay, these results
suggest that PA-6 has a higher affinity for the polar surface of
the clay than for the largely non-polar surfactant, while the
opposite is true for non-polar matrices.
Thermoplastic polyurethanes (TPUs) have similar func-
tional groups as PA-6 but rather different repeat unit structures.
There is a growing literature on polyurethane nanocomposites
describing, among other things, the effect of methods of
preparation, polyurethane and organoclay structure, and hard
segment concentration on degree of clay dispersion or exfolia-tion, microphase morphology, hydrogen bonding, rheology,
mechanical properties, thermal stability, flame retardancy,
water sorption, and barrier properties [2,14e27]. There are very
few reports on polyurethane nanocomposites prepared by melt
processing [20,21] in spite of the obvious advantages of this
approach [28e30].
The objective of this study is to explore the effect of the
surfactant chemical structure on the morphology and mechan-
ical properties of TPU nanocomposites prepared by melt pro-
cessing. Detailed comparisons of the structure of the organic
treatment (such as number of alkyl tails, saturation of the alkyl
chain, different side groups, and length of the alkyl chain) are
reported here for nanocomposites formed from a medium-
hardness TPU. A more limited study of nanocomposites based
on high-hardness TPU is also reported. Mechanical properties
and morphology characterization are used to evaluate the
structure and performance of these composites.
2. Experimental
2.1. Materials
The materialsused in this study are described in Table 1. Two
commercial grades of thermoplastic polyurethane were selected
for this work; one was a medium-hardness polyester-based TPU
from Dow Plastics and the other was a high-hardness polyether-
based TPU from BASF. The medium- and high-hardness ther-
moplastic polyurethanes will be referred to as M-H and H-H
TPUs, respectively. Several amine based organoclays from
Southern Clay Products were chosen to determine the effect
of the number of long alkyl tails, M3(HT)1 vs. M2(HT)2, satura-tion of the primary tail,M3T1 vs.M3(HT)1, the effect of hydroxy
ethyl functional groups, (HE)2M1T1 vs. M3T1, and length of the
alkyl tail, (HE)2M1T1 vs. HE2M1C1. The chemical structures
of the alkyl ammonium surfactants used to form the organoclays
are shown in Fig. 1. All data are reported in terms of weight per-
cent montmorillonite, wt% MMT, in the composite rather than
the amount of organoclay, since the silicate is the reinforcing
component. The amount of MMT was determined by heating
extruded pellets in a furnace at 900 C for 45 min and weighing
the remaining MMT ash correcting for loss of structural water
during incineration [29].
Table 1
Materials used in this study
Material (designation
used in this paper)
Supplier designation Specificationsb Supplier
Thermoplastic polyurethanes
Medium-hardness
TPU (M-H TPU)
Pellethane 9102-90AE Polyester-based TPU Dow Plastics
Methylenediphenyl diisocyanate
1,4-Butanediol
3-Caprolactone
Shore D hardness 58 DHigh-hardness TPU
(H-H TPU)
Elastollan 1174D50 Polyether-based TPU BASF
Methylenediphenyl diisocyanate
Poly(tetrahydrofuran)
Shore D hardness 73 D
Organoclaysa
M3(HT)1 Experimental: trimethyl
hydrogenated-tallow ammonium chloride organoclay
95 MER, organic content 29.6 wt%,d001 spacing 18.0 A
Southern Clay Products
M2(HT)2 Cloisite 20A: dimethyl
bis(hydrogenated-tallow) ammonium chloride organoclay
95 MER, organic content 39.6 wt%,d001 spacing 24.2 A
Southern Clay Products
M3T1 Experimental: trimethyl tallow quaternary
ammonium chloride organoclay
95 MER, organic content 29.1 wt%,d001 spacing 17.8 A
Southern Clay Products
(HE)2M1T1 Cloisite 30B: bis(2-hydroxy-ethyl)methyl
tallow ammonium chloride organoclay
90 MER, organic content 31.5 wt%,d001 spacing 17.9 A
Southern Clay Products
HE2M1C1 Experimental: bis(2-hydroxy-ethyl)methyl
coco ammonium chloride organoclay
95 MER, organic content 26.4 wt%,d001 spacing 15.2 A
Southern Clay Products
a Short hand notations for substituents on ammonium compounds: M methyl, HT hydrogenated-tallow, T tallow, HE hydroxy-ethyl, and C* coco.Tallow and coco are natural products composed predominantly of unsaturated C18 alkyl chains (65%) and C12 alkyl chains (48%), respectively.b
MERThe organic content in terms of milliequivalents of amine salt/100 g of clay.
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2.2. Melt processing
TPU nanocomposites were prepared by melt compoundingin a Haake, co-rotating, intermeshing twin screw extruder (D 30 mm, L/D 10) at a screw speed of 280 rpm, a feed rate of1200 g/h, and a temperature of 190 C. Prior to each processing
step, all materials containing thermoplastic polyurethane were
dried in a vacuum oven at 80 C for a minimum of 16 h.
Dried extruded pellets were injection molded into standard
tensile bars (ASTM D638) using an Arburg Allrounder 305-
210-700 injection molding machine. A barrel temperature
range of 200e215 C from feed to nozzle was used for the
M-H TPU materials, while a range of 180e200 C was used
for the H-H TPU materials. A mold temperature of 50 C, in-
jection pressure of 70 bar, and holding pressure of 65 bar were
used for both materials. After molding, the specimens weresealed and placed in a vacuum desiccator for a minimum of
24 h prior to mechanical testing.
2.3. Characterization
Tensile tests were conducted according to ASTM D638 us-
ing an Instron 1137 testing machine. Modulus was determined
using an extensometer at a crosshead speed of 0.51 cm/min.
Elongation at break was obtained from crosshead travel using
an effective gauge length of 9.04 cm and a crosshead speed of
5.1 cm/min. Tensile stress at 300% elongation was reported,
due to its standard use in the polyurethane literature; it will
be referred to as the 300% stress. The values reported here rep-
resent an average of five specimens for modulus, and three to
four specimens for elongation at break and 300% stress; stan-
dard deviations were typically of the order of 3% for modulus,
and 1e30% for elongation at break. The values reported for
300% stress were obtained from the most representative
stressestrain curve.
Wide-angle X-ray diffraction (WAXD) scans were made us-
ing a Scintag XDS 2000 diffractometer in reflection mode, with
an incident wavelength of 0.154 nm at a scan rate of 1.0 /min.
The analysis was performed using tensile bars for the polymer
samples while the clay was analyzed in powder form.
Transmission electron microscopic (TEM) images were ob-
tained using a JEOL 2010F transmission electron microscope
operating at an accelerating voltage of 120 kV and a Phillips
EM208 transmission electron microscope with an accelerating
voltage of 80 kV. Samples for TEM were cryogenically cut into
ultra thin sections (50e70 nm thick) with a diamond knife at
a temperature of40
C using an RMC PowerTome XL ultra-microtome with a CR-X universal cryosectioning system.
These sections were taken from the central part of a tensile
bar normal to the flow direction and viewed in the TEM parallel
to the flow direction (FD). Scanning transmission electron mi-
croscopy (STEM) images were obtained using a JEOL 2010F
transmission electron microscope operating in the dark-field,
scanning mode at an accelerating voltage of 200 kV.
The TEM images shown in this manuscript are representa-
tive of the morphologies of the nanocomposites observed
through TEM; hence, the particle analysis results from the pro-
cedure described below may not be in exact accord with the
observations from a single image since they represent averages
over a much larger sample area. The analyses were done to en-sure an objective and accurate representation of the morphol-
ogy of the nanocomposite systems.
2.4. Particle analysis
Particle analysis was done with Adobe Photoshop, using
three TEM micrographs, at 25K magnification, for each sam-
ple. The average particle length was determined by tracing
the clay particles electronically, within Adobe Photoshop,
into a transparent layer [31] similar to the procedure used in
a previous study [32]. The area of interest was also traced
into a transparent layer within Adobe Photoshop. The measure-ments of particle length and area were determined using
IPTK 5.0, a commercially available set of plug-ins, for use
with Adobe Photoshop, developed by Reindeer Graphics Inc.
[33]. The average particle density was considered as the aver-
age number of particles per mm2. The specific particle density
is the average particle density normalized to a specific clay con-
centration (in this study, the specific clay concentrations will be
1 and 5.5 wt% MMT). These values are a measure of the extent
of exfoliation of the organoclay in the polymer matrix.
3. Results and discussion
3.1. Effect of organic modifier for the M-H TPU
The following results show the effect of organoclay structure
on morphology and properties of nanocomposites prepared
from the medium-hardness TPU. Morphology, as judged by
WAXD, TEM, particle analysis, and dark-field STEM, and
mechanical properties, assessed by Youngs modulus, 300%
stress, and elongation at break, are used to characterize the
structureeproperty relationship of these materials.
3.1.1. Clay dispersion
Fig. 2 shows WAXD scans for various nanocomposites made
in this study containingw2.3 wt% MMT, along with a scan of
(a) One-tail
(b) Two-tail
(c) Methyl
(e) Hydroxy ethyl. short tail
(d) Hydroxy ethyl. long tail
N+
CH3
CH3
CH3
M3(HT)1
HT(C18)
CH3
M2(HT)2
N+HT(C18)
HT(C18)
CH3
CH2CH2OH
CH2CH2OH
(HE)2M1T1
T(C18) N+
CH3
CH2CH2OH
CH2CH2OH
N+
(HE)2M1C*1
C*(C12) CH3
CH3
N+
T(C18) CH3
CH3
M3T1
Fig. 1. Molecular structure and nomenclature of the amines used to form the
organoclays used in this study.
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each organoclay for comparison. In every case, the nanocompo-
site scans show basal reflections indicatingthat all the nanocom-
posites contain tactoids of organoclay with high enough order
and concentration to diffract X-rays. There is an increase in
the d-spacing of all the nanocomposites with respect to that of
the pure organoclays, except for the samples containing the
HE2M1C1 surfactant. The increase in d-spacing for the nano-
composites made with M3(HT)1 is from 1.8 to 2.9 nm, while
those for M2(HT)2 shows a slightly smaller increase from 2.55
to 3.41 nm. M3T1 shows an increase in d-spacing from 1.75 to
2.96 nm and (HE)2M1T1 shows a weaker and more diffuse peak
that seems to be centered around 3.05 nm, with respect to
1.77 nm of the organoclay. According to conventional interpre-
tations, this suggests some degree of polymer intercalation into
the organoclay galleries. It should be noted that the WAXD pat-
terns in Fig. 2 show only one basal reflection for the organo-
clays, while more high-order reflections were observed for the
nanocomposite samples. The organoclay scans were made from
powders while the nanocomposite scans were made from the
surface of injection molded samples. The greater orientations
in the latter lead to multiple reflections while the more random
orientations of the organoclay powders do not.
TEM micrographs shown in Figs. 3 and 4 provide addi-
tional insight about the morphology of these nanocomposites.
At low concentrations, all the composites have a similar struc-
ture, except for the sample containing (HE)2M1T1, which
shows a noticeably higher number of particles. At high con-
centrations, the nanocomposites from (HE)2M1T1 also have
the highest number of particles with the smallest size. Thenanocomposites containing M3(HT)1 and M3T1 have a some-
what lower number of particles of slightly larger size; while
the samples containing HE2M1C1 show a mixture of small
particles and larger agglomerates. The two-tailed organic
modifier (M2(HT)2) produces the lowest number of particles
with the largest size. At the same clay concentration, the par-
ticle count is an indicator of the extent of exfoliation of clay
platelets in the polymer matrix. Samples with a low particle
count have large particles with high platelet agglomeration
and, as the particle count increases, the particle size decreases
as a result of the breakup of large particles, producing better
dispersion.
The number of long alkyl tails on the surfactant in theorganoclay affects the morphology of these TPU nanocompo-
sites (Fig. 3a and b). M2(HT)2 leads to a small number of large,
extended tactoids, while M3(HT)1 produces a higher number of
small, elongated tactoids. The unsaturated surfactant, M3T1,
produces a very similar morphology as that obtained from the
saturated organoclay, M3(HT)1, but there seems to be a larger
number of particles and more alignment in the nanocomposites
made from the saturated organoclay (Fig. 3b and c). The sur-
factant with hydroxy ethyl functional groups, (HE)2M1T1,
leads to the smallest tactoids and many particles are single
platelets. There is a higher particle count, or a higher degree of
clay dispersion in the samples containing hydroxy ethyl groupsthan in the ones containing methyl substituents (Fig. 4a and b).
The length of the alkyl tail also seems to affect clay dispersion,
as seen in Fig. 4b and c; the shorter alkyl tail does not produce
as high clay dispersion as the longer alkyl.
These qualitative trends in Figs. 3 and 4 are quantified by
particle analysis of TEM images; the results are shown in
Table 2. The complex shapes of the clay particles in TPU nano-
composites made thickness estimation very difficult, so values
of aspect ratios for these samples could not be obtained. Nev-
ertheless, measurements of particle lengths and densities give
insight about the degree of platelet dispersion of these nano-
composites. At low concentrations, the particle lengths are
very similar for all the samples, while the specific particle den-
sity is highest in the samples made with (HE)2M1T1, as seen
in the TEM micrographs. At high concentrations, the samples
made from M2(HT)2 have the highest particle length, while the
ones made from (HE)2M1T1 have the lowest. The specific par-
ticle density is again highest for the nanocomposites made
from the (HE)2M1T1 organoclay, suggesting the highest degree
of dispersion, while M2(HT)2 produces an extremely low par-
ticle density consistent with the large agglomerates seen in
TEM. High matrix reinforcement might be expected from a
high particle density; however, the aspect ratio of the filler
particles is the true parameter needed to assess the degree of
reinforcement.
2
2 4 6 8 10
(HE)2M1C*1
(HE)2M1C*1/M-H TPU
(HE)2M1T1
M3(HT)1/M-H TPU
M3(HT)1
M2(HT)2/M-H TPU
M2(HT)2
M3T
1
M3T1/M-H TPU
(HE)2M1T1/M-H TPU
~2.3 wt.% MMT
Shifted
Intensity
[cps]
Fig. 2. WAXD patterns for pure organoclays and their respective M-H TPU
nanocomposites containingw2.3 wt% MMT.
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Fig. 3. TEM micrographs for nanocomposites based on M-H TPU and organoclays: (a) M2(HT)2, (b) M3(HT)1, and (c) M3T1; with concentrations ranging from
0.48 to 0.94 wt% MMT (above) and from 4.61 to 6 wt% MMT (below).
(a) M3T1 (b) (HE)2M1T1 (c) (HE)2M1C*1
200 nm 200 nm
200 nm
200 nm
200 nm 200 nm
Fig. 4. TEM micrographs for nanocomposites based on M-H TPU and organoclays: (a) M3T1, (b) (HE)2M1T1, and (c) HE2M1C1; with concentrations ranging
from 0.9 to 0.95 wt% MMT (above) and from 5.46 to 6 wt% MMT (below).
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The complex shapes of the clay particles in TPU nanocom-
posites were explored by comparing bright-field TEM and
dark-field STEM images. When using the dark-field STEM
technique, the MMT platelets appear white, while the polymer
matrix, holes in the specimen, or any other low-atomic-density
constituent are seen as a black background. Fig. 5 confirms
Table 2
Particle analysis results of M-H TPU nanocomposites
Organoclay
nanocomposites
No. of particles
analyzed
Particle length (nm) Particle density
(particles/mm2)
Specific particle density
(particles/mm2)Number average Weight average
M3(HT)10.48 wt% MMT 159 138 3 172 11 8 2 16 4
5.05 wt% MMT 842 141 7 183 12 43 4 47 5M2(HT)2
0.61 wt% MMT 233 113 7 144 7 11 3 19 44.61 wt% MMT 182 215 31 327 31 10 1 12 1
M3T10.94 wt% MMT 541 142 6 180 12 9 1 10 16.00 wt% MMT 1311 191 7 248 16 22 1 21 1
(HE)2M1T10.9 wt% MMT 946 114 10 142 10 28 4 31 55.46 wt% MMT 1376 118 2 149 2 93 12 93 12
HE2M1C1
0.95 wt% MMT 546 135 7 184 19 10 1 10 15.68 wt% MMT 897 133 12 132 17 56 7 54 7
(c) Medium-Hardness TPU/(HE)2M1C*1Dark Field STEM images
(a) Medium-Hardness TPU/M2(HT)2 (b) Medium-Hardness TPU/(HE)2M1C*1
100 nm
100 nm0.2 m
200 nm
Fig. 5. Dark-field analysis of M-H TPU nanocomposite particles. Above: bright-field TEM images of M-H TPU nanocomposites (low magnification many particles
(left) and high magnification showing one particle (right)). Below: dark-field STEM images of M-H TPU nanocomposites (low magnification showing many
particles (left) and high magnification showing one particle (right)).
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that the small grey areas seen around some of the particles in
the bright-field TEM images are in fact clay. In a specific ex-
ample, Fig. 5c shows that the grey area surrounding the parti-
cle in Fig. 5b is white, which suggests the presence of one or
more MMT platelets. In bright-field TEM, sharp lines show
platelet edges, i.e., the platelets are aligned with the electron
beam, while grey areas of different intensities show one or
more skewed or misaligned platelets. A careful analysis of
the TEM images has led us to suggest a series of possible
platelet orientations within the microtomed section that mightexplain the clay morphologies seen by TEM. Images contain-
ing particles with a grey area on only one side of a sharp line
may be the result of a platelet oriented within the microtomed
section as suggested in Fig. 6; the sharp line being the edge of
the platelet aligned with the electron beam, and the grey area
being the skewed body. Particles with grey areas on both sides
of sharp edges suggest more complex configurations of what is
described in Fig. 6. Figs. 7 and 8 illustrate how grey areas on
both sides might involve having a particle with one or many
platelets separated on the far side of the section, or a particle
with many platelets skewed towards one side. As shown be-
fore, TEM images of some TPU nanocomposites have parti-
cles containing sharp dark areas in the center, surrounded bylarge grey areas. It is proposed that these images represent par-
ticles with aligned platelets in the center, surrounded by
a smaller number of disordered platelets with no edges on
the electron beam path. TEM micrographs taken with the
view parallel to the normal direction (ND) (i.e., in the flow di-
rectionetransverse direction (FDeTD) plane) show the same
morphology, indicating that this condition might be present
in all directions (Fig. 9).
3.1.2. Mechanical properties
Fig. 10 shows representative stressestrain curves for the
nanocomposites prepared in this study and illustrate three
distinctive types of behaviors. Addition of clay produces an in-
crease in the modulus at small strains in all cases; however, the
different organoclays affect the large strain behavior in differ-
ent ways and may even produce limitations on elongation. In
the stressestrain curve for the nanocomposites made from
(HE)2M1T1, the stress is increased at all strains as the concen-
tration of clay increases (Fig. 10a). This was also observed for
the stressestrain curves of the nanocomposites made with
HE2M1C1, and M3T1, but to a lesser extent. The opposite
behavior is observed in the stresse
strain diagram for the nano-
composites made with M2(HT)2 (Fig. 10c), as the clay concen-
tration is increased, the stress levels at high strains are
z
y
x
z
y
x
y
x
z
y
x
z
y
x
z
y
x
z
y
x
y
x
y
x
TEM
TEM
specimen
micrograph
Lateral
view of
specimen
Fig. 6. Schematic illustration of possible platelet orientation for a TEM imageof a clay particle with a grey area on one side of a particle.
z
y
x
z
y
x
y
x
specimenTEM
TEMmicrograph
representation
Lateralview of
specimen
TEM
micrograph
Fig. 7. Schematic illustration of possible platelet orientation for a TEM imageof a clay particle with grey areas on both sides of the particle: particle with one
or many platelets separated on the far side of the sample.
z
y
x
y
x
z
y
x
TEM
specimen
TEM
micrograph
representation
Lateral
view of
specimen
TEM
micrograph
Fig. 8. Schematic illustration of possible platelet orientation for a TEM image
of a clay particle with grey areas on both sides of the particle: particle with
many platelets, all skewed towards one side.
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reduced. An intermediate behavior is seen for the nanocompo-
sites made with M3(HT)1, where the stress at low clay concen-
trations is higher than that of the pristine TPU, but lower at
high clay concentrations (Fig. 10b). This suggests that at
high strains, the (HE)2M1T1 organoclay continues to reinforce
the TPU even at high clay concentrations, M3(HT)1 provides
high strain reinforcement only at low clay concentrations,
while M2(HT)2 does not reinforce the polyurethane matrix
at high strains. The elongated tensile bars show signs of
permanent set and stress whitening; both increase as the clay
concentration increases. The permanent set is highest for the
samples made from M2(HT)2, followed by the ones made
with M3(HT)1. Samples made with (HE)2M1T1, M3T1, and
HE2M1C1 all have similar permanent set, lower than the
samples made with M2(HT)2 and M3(HT)1. On the other
hand, stress whitening is highest for the samples made with
(HE)2M1T1, followed by the samples made with M3T1 and
HE2M1C1. The samples with M3(HT)1 show somewhat
View parallel to the normal direction (ND)(TD-FD Plane):
FD
ND
TD
View
FD
View
ND
TD
View parallel to the flow direction (FD)
(ND-TD Plane):
Normal direction
(ND)
Flow direction
(FD)
Transverse direction
(TD)
Mold filling
direction
(view parallel to ND)Medium-Hardness TPU/M2(HT)2
200nm
Fig. 9. TEM sample orientations used in this work and TEM image of TPU/
M2(HT)2 nanocomposites (viewed parallel to the normal direction).
Pure M-H TPU
Pure M-H TPU
(b) M3(HT)1
(c) M2(HT)2
Strain [%]
0 100 200 300 400 500
Strain [%]
0 100 200 300 400 500
Strain [%]
0 100 200 300 400 500
Stress
[MPa]
0
5
10
15
20
25
30
Stress[MPa]
0
5
10
15
20
25
30
Stress[MPa]
0
5
10
15
20
25
30
Pure M-H TPU
1 wt.% MMT
5.5 wt.% MMT
2 wt.% MMT
5 wt.% MMT
0.5 wt.% MMT
4.5 wt.% MMT
(a)(HE)2M1T1
Fig. 10. Stressestrain behavior of M-H TPU and its nanocomposites: (a)
(HE)2M1T1, (b) M3(HT)1, and (c) M2(HT)2.
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less stress whitening, while those made with M2(HT)2 have the
lowest stress whitening. A more detailed study would be
needed to sort out all the operative mechanisms behind these
observations, but this is beyond the scope of this work.
Figs. 11 and 12 show the Youngs modulus and stress at
300% strain, respectively, of the nanocomposites prepared in
this study. The addition of clay produces an increase in theYoungs modulus of all the nanocomposites, even at low load-
ings. As the clay concentration increases, the nanocomposites
made from (HE)2M1T1 produce the largest increase in Youngs
modulus, while the ones made from M2(HT)2 present the
smallest. Fig. 11a shows that the one-tailed organoclay leads
to higher stiffness values than the two-tailed organoclay, while
there is not much difference in the Youngs modulus for nano-
composites based on the saturated and unsaturated organo-
clays. There is a significantly higher increase in Youngs
modulus of samples made from the organoclay containing
(HE)2M1T1 (Fig. 11b), suggesting that the hydroxy ethyl func-
tional groups and a longer alkyl tail lead to greater TPU matrix
reinforcement.Fig. 12 shows that the stress at 300% elongation is highest
in samples made from (HE)2M1T1, while M2(HT)2 produces
the lowest 300% stress overall. M3T1 and HE2M1C1 produce
a substantial increase in the 300% stress, even at high clay
concentration, as opposed to the behavior observed for
M2(HT)2, where it gradually decreases with MMT concentra-
tion. On the other hand, the 300% stress for the samples madewith M3(HT)1 increases slightly at low loadings and then re-
mains almost constant. It should be noted that the 300% stress
for the nanocomposites containing (HE)2M1T1 is presented
only at low concentrations, since the elongation at break is
less than 300% for composites with higher concentrations.
The effect of organoclay structure on elongation at break
could not be fully evaluated in this study since most of the
samples prepared did not break before the Instron machine
limit of 400% elongation. The only sample that broke before
the 400% limit was the highest concentration sample made
from (HE)2M1T1. Reports in the literature show somewhat
mixed results regarding the elongation at break. Some reportsshow an increase in elongation at break with increasing clay
concentration and dispersion [2], some show a decrease
[20e22], while others show a maximum of ultimate tensile
properties at a low clay concentration (some mention 1 wt%
and others mention 3 wt% of clay) [14e17,19,21,24,25].
The evidence described above suggests that the organoclay
containing one long alkyl tail leads to better dispersion of
MMT platelets and greater matrix reinforcement than the or-
ganoclay that contains two long alkyl tails. It was also seen
that the unsaturated organoclay produces a very similar struc-
ture to that obtained from the saturated organoclay, which sug-
gests that hydrogenating the tallow tail has no significant
effects on morphology and mechanical properties. This was
also found for PA-6, where Fornes et al. proposed that polar
polymers like polyamides, and apparently polyurethanes,
have a relatively good affinity for the polar clay surface and
that the single tail represents the best balance between reduc-
ing the plateleteplatelet attraction and allowing the polymer
to have access to the silicate surface; two alkyl tails limit
the access of the polymer chains to the clay surface to a greater
extent [10]. It was also suggested that the saturated and unsat-
urated organic treatments may give very similar morphologies
and degrees of reinforcement because they provide almost the
same access of the polymer matrix to the clay surface. Evi-
dently, the TPU matrix interacts favorably with the silicate
wt.% MMT
(b)
0 1 2 3 4 5 6 7
wt.% MMT
0 1 2 3 4 5 6 7
Young'sModulus[MPa]
40
60
80
100
120
140
160
Young'sModulus[MP
a]
40
60
80
100
120
140
160
M3(HT)1M3T1
M3T1
M2(HT)2
(HE)2M1T1
(HE)2M1C*1
(a)
Fig. 11. Youngs modulus of M-H TPU nanocomposites. (a) M3(HT)1,
M2(HT)2, and M3T1 and (b) M3T1, (HE)2M1T1, and HE2M1C1.
300%S
tress[MPa]
5
10
15
20
25
M3T1
M3(HT)1
M2(HT)2
(HE)2M1T1 (HE)2M1C*1
wt.% MMT
0 1 2 3 4 5 6 7
Fig. 12. 300% stress for M-H TPU nanocomposites.
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surface but not as well as PA-6, since the latter gives much
higher platelet exfoliation. On the other hand, the results pre-
sented here also show that the organoclay with hydroxy ethyl
functional groups leads to significantly higher clay dispersion
and platelet exfoliation, producing a greater stiffness than the
corresponding organoclay without these functional groups.
This result deviates from what was seen for PA-6, where orga-noclays having methyl functional groups produce a much
higher reinforcement. Fornes et al. suggested that the hydroxy
ethyl groups provide some degree of shielding of the clay sur-
face, where the eOH moiety may be attracted towards the
polar clay surface [10]. Similar observations were reported
for non-polar polymers, where organoclays that provide
a higher shielding of the clay surface, i.e., M2(HT)2 and
(HE)2M1T1, gave better clay dispersion and matrix reinforce-
ment [12]. Recently, Choi et al. showed that nanocomposites
made from hydroxylated polyisoprene-block-polystyrene-
block-polybutadiene (ISBOH) triblock copolymers had a
higher degree of clay dispersion with (HE)2M1T1 organoclay
than those made from M2(HT)2-125. It was proposed thatthe presence of attractive interactions between the hydroxyl
group of the ISBOH triblock copolymer and the hydroxyl
group in the organoclay aided clay dispersion [34]. It seems
like the hydroxy ethyl functional groups in the organoclay
have the possibility to form hydrogen bonds with either the
surface of the clay or the polar groups in the polymer matrix.
In the case of TPU nanocomposites, we propose that the high
affinity of the polymer matrix to the hydroxy ethyl groups in
the organoclay might make hydrogen bonding with the poly-
mer matrix more favorable, aiding clay dispersion and exfoli-
ation. Reports in the literature have shown in general, that
organoclays containing hydroxy ethyl functional groups leadto higher silicate dispersion and delamination in polyurethanes
[16,18,20,21,24,25]. It is suggested that the hydroxy ethyl
groups may match the polarity of the polyurethane matrix bet-
ter than just having hydrocarbon chains, making dispersion
more favorable [18].
3.1.3. Color formation
Fornes et al. and Yoon et al. [35,36] have described color
formation for PA-6 and polycarbonate (PC) nanocomposites
made with different organoclays. They showed that the level
of color intensifies with clay content and that color formation
depends on the nature of the organoclay used. Their study also
indicates that organoclays with unsaturated surfactants lead to
more color depth than organoclays with saturated structures,
and that a deeper color was observed for the surfactant con-
taining both hydroxy ethyl and unsaturated tallow. This is
also true for these TPU nanocomposites. Fig. 13 shows how
the color of M-H TPU nanocomposites varies with organoclay
structure. This figure shows that all the samples have high op-
tical transparency, as observed by Chang and An [17], and that
organoclays containing a hydrogenated-tallow surfactant pro-
duce samples with a yellow tint, hydroxy ethyl functional
groups produce a brown tone, while the unsaturated tallow sur-
factant generates a green hue. Fornes et al. and Yoon et al.
mention that color formation is a product of clay-induced
chemical reactions, such as matrix degradation and/or surfac-
tant stability, and that a higher dispersion leads to a greater ex-
posure of the clay surface and, hence, a greater opportunity for
chemical reaction [35,36]. PA-6 and PC nanocomposites only
show shades of yellow and brown, while some TPU nanocom-
posites show shades of green as well. A clay purification ex-
periment was done to determine if the greenish hue was
produced by clay impurities. M3T1 organoclay was purifiedwith methanol, and was later used to make TPU nanocompo-
sites. The same green tint was observed in these samples, so
we conclude that the green color formation is not due to
clay impurities. Taking this into account, we suggest that the
different colors may arise from specific chemical interactions
between the specific organic treatment and the polymer mate-
rial or additives therein.
3.2. Structure and properties for the H-H TPU
Thermoplastic polyurethanes are composed of hard and soft
segments, formed by aromatic isocyanates and long-chain di-
ols. These segments segregate, like in semi-crystalline poly-
mers, having a direct effect on the properties of the material.
The long-chain diols are mainly polyether- or polyester-based,
and the type of long-chain diol used also affects the properties
of the final TPU. Reports in the literature suggest that the
structure of TPU materials, such as type of polyol and ratio
of soft to hard segments, affects the mechanical properties
and morphology of their nanocomposites [16,20].
A series of nanocomposites were made from the high-
hardness TPU described earlier and (HE)2M1T1 and M3(HT)1organoclays. The results are compared to the nanocomposites
made from the medium-hardness TPU described in the previous
section in an effort to explore the effect of different types of TPU
(f)
(e)
(d)
(c)
(b)
(a)
Fig. 13. Color formation of M-H TPU nanocomposites. (a) pure M- H TPU,(b)e(f) M-H TPU nanocomposites with w5.5 wt% MMT: M3(HT)1,
M2(HT)2, M3T1, (HE)2M1T1, and HE2M1C1 organoclays, respectively.
(For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
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materials on morphology and properties of nanocomposites.
The high-hardness TPU is a polyether-based TPU with a Shore
D hardness of 73 D, while the medium-hardness TPU is a poly-
ester-based TPU with a Shore D hardness of 58 D. The mono-
mer precursors for the high-hardness TPU are MDI and
polytetrahydrofuran, while the medium-hardness TPU is made
from MDI, 1,4-butanediol, and 3-caprolactone. The differencesin these TPUs were evidenced, in part, during the injection
molding of these materials and its nanocomposites. The high-
hardness materials required lower barrel temperatures, shorter
injection times, longer cooling times, and a lower mold tem-
perature for injection molding. For this TPU, no TEM images
or mechanical testing are shown at high MMT concentrations
since such samples could not be produced because the current
injection molding machine could not achieve the low molding
temperatures needed.
3.2.1. Clay dispersion
WAXD patterns in Fig. 14 show basal reflections for the
H-H TPU nanocomposites, indicating the presence of tactoids.
The d-spacing for the medium-hardness TPU, as mentioned
before, increases from 1.77 to 3.05 nm and from 1.8 to
2.9 nm for hydroxy ethyl and one-tailed organoclays, respec-
tively; while the H-H TPU nanocomposites show a low broad
peak at 3.25 nm and an intense peak at 3.22 nm for
(HE)2M1T1 and M3(HT)1 organoclays, respectively.
Fig. 15 compares high magnification TEM images of these
nanocomposites based on the two types of TPU. In both cases,
the number of particles is higher for the samples made with
hydroxy ethyl organoclay, where the particles are smaller in
size and a larger number of single platelets exist. Even though
the MMT concentration of the TEM micrograph for the M-HTPU/M3(HT)1 sample is lower than the MMT concentration of
the other micrographs, particle analysis results shown previ-
ously show that the specific particle density of M-H TPU/
M3(HT)1 nanocomposites at 1 wt% MMT is significantly
lower than those made with (HE)2M1T1, which is in accord
with our observations.
3.2.2. Mechanical properties
The stressestrain behavior of the M-H and H-H TPU nano-
composites with (HE)2M1T1 organoclay is compared in
Fig. 16. The stressestrain diagrams of the pure H-H TPU and
its nanocomposites have a very different shape than those from
M-H TPU; a yield point is observed for the H-H TPU mate-
rials, while no distinct yield point is observed for the M-HTPU nanocomposites. Regardless of clay concentration, the
stress levels for the H-H TPU samples are much higher than
that of the M-H TPU samples. H-H TPU samples, unlike the
M-H TPU samples, show a yield point, after which a distinct
drop in tensile stress is observed indicating the onset of
necking. As the clay concentration increases the stress in-
creases, similar to the behavior observed for the M-H TPU
nanocomposites.
Fig. 17 shows the Youngs modulus of the nanocomposites
made from H-H TPU at different MMT concentrations. The
moduli of the H-H TPU matrix and its composites are much
higher than the corresponding M-H TPU matrix and compos-
ites. Nevertheless, both TPUs have the same response to orga-noclay structure, i.e., better reinforcement is obtained with the
hydroxy ethyl organoclay. The medium-hardness TPU shows
a large increase in modulus with increasing wt% MMT, while
in the high-hardness samples, (HE)2M1T1 shows almost no en-
hancement until 1 wt% MMT and almost no enhancement at
all for the one-tailed organoclay. This difference in modulus
enhancement may be due in part to the considerably lower
modulus of the M-H TPU matrix. As stated by Fornes et al.
low-modulus matrices offer a higher potential for reinforce-
ment than high-modulus matrices, for a given filler aspect
ratio, as a result of the larger ratio of filler to matrix moduli.
As this ratio increases, the nanocomposite modulus becomesmore sensitive to the aspect ratio of the filler, so a larger
increase in modulus may not suggest a higher degree of exfo-
liation or reinforcement [37]. This was also mentioned by
Finnigan et al. for TPU nanocomposites [20].
The elongation at break also shows significant differences
between the two types of TPUs. H-H TPU nanocomposite
samples show noticeable necking, and the elongation at break
was lower than the 400% machine limit (Fig. 18). The elonga-
tion at break in the samples made with the H-H TPU matrix
remains almost constant for the one-tailed organoclay while
it decreases significantly with the hydroxy ethyl organoclay.
This is consistent with the levels of reinforcement obtained
with these two organoclays. Since the elongation at break of
the M-H TPU could not be fully explored due to machine lim-
itations, we cannot give any conclusions regarding the effect
of the different types of TPUs on elongation at break. Tien
and Wei show an increase in Youngs modulus and a
decrease in elongation at break with increasing hard segment
concentration [16]; the results here show an indication of
similar behavior.
Overall, both types of TPUs show the same trends with
respect to organoclay structure, e.g., better clay dispersion and
enhancement of mechanical properties are observed for the
organoclay containing hydroxy ethyl functional groups as com-
pared to the organoclay with one-tailed hydrogenated-tallow
2
2 4 6 8 10
ShiftedIntensity[c
ps]
M3(HT)1/M-H TPU
M3(HT)1
(HE)2M1T1
(HE)2M1T1/M-H TPU
H-H TPU
~2 wt.% MMT
Fig. 14. WAXD patterns of M3(HT)1 and (HE)2M1T1 organoclays and its high-
hardness TPU nanocomposites.
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substituents. This supports the reasoning made in the previous
section where it was proposed that the hydroxy ethyl func-
tional groups lead to higher degrees of clay dispersion of TPU
nanocomposites. Apparently, the soft to hard segment ratio
and/or polyol structure have a noticeable effect on the
morphology and mechanical properties of TPU nanocompo-
sites. H-H TPU nanocomposites showed a slightly higher
number of particles and clay dispersion, i.e., better exfoliation.
However, adding clay to the M-H TPU causes a greater
improvement in relative modulus, e.g., at a concentration of
Medium-Hardness TPU High-Hardness TPU
0.5 wt.% MMT 1 wt.% MMTM3(HT)1
1 wt.% MMT 1 wt.% MMT(HE)2M1T1
100 nm
100 nm 100 nm
100 nm
Fig. 15. TEM micrographs of medium-hardness and high-hardness TPU nanocomposites containing (HE) 2M1T1 and M3(HT)1 organoclays.
Strain [%]
0 100 200 300 400 500
Stress[MPa]
0
10
20
30
40
50
60
M-H pure
H-H pure
H-H3 wt.% MMT
M-H2 wt.% MMT
M-H1 wt.% MMT
H-H1 wt.% MMT
(HE)2M1T1
Fig. 16. Stressestrain curves for medium-hardness and high-hardness TPUs
and its nanocomposites containing (HE)2M1T1 organoclay.
wt.% MMT
0 1 2 3 4 5 6 7
Young'sModulus[GP
a]
0.6
0.8
1.0
1.2
1.4
(HE)2M1T1
M3(HT)1
H-H TPU
Fig. 17. Youngs modulus of high-hardness TPU nanocomposites containing
(HE)2M1T1 and M3(HT)1 organoclays.
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w2 wt% MMT, the relative modulus for (HE)2M1T1 nano-composites made from the M-H TPU is 1.57, while it is
1.13 for the nanocomposites made from the H-H TPU. The
primary reason for the greater relative modulus in the case
of M-H TPU nanocomposites is due to the much lower abso-
lute modulus of the M-H matrix, as predicted by composite
theory [37].
Self-consistent field theory calculations suggest that the
polar hard segments are attracted towards the silicate surface
while the non-polar soft segments separate the individual
platelets to regain entropy [38]. The structure of the TPU
has an effect on the polarity of the polymer molecules, and dif-
ferent levels of polarity influence the affinity between thepolymer and the organoclay. It is proposed here that these
two materials interact differently with the organoclay, and
the structure of the TPU may lead to differences in clay disper-
sion and relative mechanical property enhancement; however,
at this point, it is not possible to be more definitive about the
underlying causes for this difference.
4. Conclusion
Structureeproperty relationships for TPU nanocomposites
prepared by melt processing from a series of alkyl ammo-
nium/MMT organoclays and medium-hardness and high-
hardness thermoplastic polyurethanes are presented here. The
structure of the organic modifier used to form the organoclay
was systematically varied to determine the effect of specific
functional groups on the degree of clay dispersion and matrix
reinforcement for M-H TPU nanocomposites. Morphology
analysis and mechanical property results show that overall,
the (HE)2M1T1 surfactant produces the best dispersion of or-
ganoclay particles and the highest matrix reinforcement, while
the M2(HT)2 surfactant produces the lowest. More specifically,
the results show (a) one long alkyl tail on the ammonium ion
rather than two, (b) hydroxy ethyl groups on the amine rather
than methyl groups, and (c) a longer alkyl tail as opposed to
a shorter one leads to higher clay dispersion and stiffness
enhancement for TPU nanocomposites. Most of these trends
are similar to what was observed in PA-6 based nanocompo-
sites, which suggests that polar polymers like polyamides,
and apparently polyurethanes, have a relatively good affinity
for the polar clay surface; and in the case of polyurethanes,
the high affinity of the matrix for the hydroxy ethyl functional
groups in the (HE)2M1T1 organoclay aids clay dispersion andexfoliation. This suggests that an organic treatment that pro-
vides the best balance between reducing plateleteplatelet at-
traction and increasing organoclayepolymer affinity will
produce a higher degree of clay dispersion. A more limited
study comparing a medium-hardness polyester-based TPU and
a high-hardness polyether-based TPU shows that both types of
TPUs exhibit similar trends with respect to organoclay struc-
ture, supporting the previous reasoning. Apparently the soft to
hard segment ratio and/or polyol structure have a noticeable
effect on the morphology and mechanical properties of TPU
nanocomposites. H-H TPU nanocomposites showed a slightly
higher number of particles and clay dispersion, i.e., better ex-
foliation. However, because of the much lower modulus of theM-H TPU, adding clay to this matrix causes a greater im-
provement in relative modulus. Finally, it should be noted that
even though some organoclays lead to TPU nanocomposites
with high degrees of clay dispersion and platelet exfoliation,
none exhibited degrees of exfoliation as high as those
observed in PA-6 nanocomposites. Nevertheless, this study
provides insights about the effect of the organoclay structure
and polarity of the polymer matrix on morphology and
mechanical properties of polymereclay nanocomposites.
Acknowledgements
This work was funded by the Air Force Office of Scientific
Research and by the Consejo Nacional de Ciencia y Tecnolo-
ga (CONACyT) of Mexico. The authors would like to thank
Southern Clay Products Inc. for providing the clay materials
and WAXD analyses. We also acknowledge Dr. Desiderio
Kovar and Dr. John Mendenhall for several helpful
discussions.
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